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This presentation is delivered by the Stanford Center for Professional 

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Development. 

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Alrighty, welcome back. Wow, that's 

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pretty loud. 

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Welcome back to CS106a. I hope I didn't just shatter your eardrums. 

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And thanks for making it out in the rain. I think the rain might have stopped a few 

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people from making it today. But, actually, today is one of the most important lectures 

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of the whole quarter, 

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so it's too bad that it would happen to happen that way. 

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So a couple of quick announcements: One is that there's two handouts, one on coding 

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style, and one on the use of variables. I'd encourage you to read both of them because 

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they are both extremely critical concepts in this class. There are some things that - it's like, 

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"Yeah, it's not so important." These two are really important so please make sure to read 

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the handout. 

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There is a couple of new concepts I want to show you briefly in the beginning, and 

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then we're gonna begin to put a whole bunch of things together. 

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So so far in class, what we've done is, when you saw Caroline we did a bunch of 

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stuff with Java so far, we've shown you a bunch of bits and pieces of 

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things and 

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put some larger pieces together. And today's the day where it really all comes 

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together, 

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and a whole bunch of stuff that we've talked about before, where I said, "Oh, later 

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on this'll make more sense," 

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today's the day when hopefully that will all make sense. 

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So first thing to cover, just very briefly, 

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is something called, "Strings." 

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And if you've been reading along in the book you've seen some references to 

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Strings. We haven't talked about them in class, and now it's time that we spent a little bit of 

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time talking about Strings. In about another week and a half we'll spend a whole bunch of 

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time talking about Strings, 

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but you should at least get a little introduction to them. 

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So what is a String? 

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A String is just a type, like we had ints and we had doubles, and 

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those boolean, stuff like that. String, except String starts with a 

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capital "S," it's not lower case, is actually a type, and so we can declare 

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variables of this type; like, we can have some variable called "str," which is a 

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String. 

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And what does a String hold? What a String really sort of means is a string 

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of characters, it's a piece of text is all a String is. 

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And the way we usually denote a String is its inside double-quotes, or 

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the 

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" " that we're used to. So we might have some String, and initialize it 

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by setting it equal to, "Hello, space, there." So it's perfectly fine to have 

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spaces and other kinds of characters inside of a String. 

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And, basically, it's just a piece of text that says, "Hello there." It's just one 

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variable that happens to have this value, "Hello there." 

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And so the important thing is it's text enclosed in double quotes. 

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And you can think of the text that you actually assign to the String. You can use 

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things on it like concatenation, just like you did in a print lin, when you 

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actually want to print some stuff on the screen, we use the plus operation to 

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concatenate pieces of text together. 

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Well, we can do that with Strings. So we could have some String, 

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we'll call this String, "name." And we might set name = "Bob." 

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And we might have some integer age, 

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which is just set to "20." And then we might have some other String, 

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so some other String s, which we want to set to be something like "name:+," 

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and then whatever the name actually is in this other String, so name. 

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What that'll do is concatenate the word Bob 

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onto name:. And then maybe we want to concatenate onto that something like 

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age, 

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and then concatenate onto that the value of the variable age. And so what we'll get here is we'll 

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get something that in the end 

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s is just some variable, so it's just some box that contains something. And what it 

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will contain is 

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"name:" 

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then concatenated on with whatever name evaluated to. Well, name:, when we look it up 

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in the box is just Bob, so we'll say, "name: Bob." 

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And then 

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concatenate onto space "age:," 

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and then the value of whatever age was, which is "20." Just like 

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this would work in a print lin, if you were to say print line, 

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and have parens around this whole thing, you would expect it to print this out. 

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You could actually just assign that to a String. 

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So same sort of concatenation works. And then you might wonder, how 

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do you assign these Strings or get these Strings? 

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You can assign what we refer to as "literal," which is an actual value to 

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the String. 

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You can build up a String by concatenating a bunch of other stuff 

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together. 

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Sometimes you want to read in some text from the user, and there's a little 

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method called "readLine," just like read int and read double, 

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where you give it some piece of text but it's gonna display 

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a question mark because it's gonna ask for something. And what it gives you 

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back is basically the whole line that the user typed in. So it's not just 

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looking for an int or a double, 

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but if they type in a whole line of text, including characters, 

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it gives you that back as a String, that's what it returns, and you can 

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assign it somewhere. Like some variable we might call "line," that's a type String. 

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So you should just, at this level, see what a String is, understand it, be able to get 

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them from users, in about a week and a half we'll do a whole bunch of funky things 

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with Strings and 

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get into the nitty-gritty. But at this point, this is just to give you a little bit 

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of an idea as to what they are because we're gonna actually use them later on 

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today. 

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And there are some references in the book that hopefully will make a little bit 

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more sense, but up until now, we didn't actually need them. So we just kind of 

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deferred the discussion. Is 

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there any questions about String? All right. So 

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the next big thing, here is the real big topic that we're gonna cover 

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today, 

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is writing our own "classes." 

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So this whole time, when we've been writing classes, so we would write something 

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like, "My Program," and it would extend Console Program, and we'd have all of our 

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methods and stuff inside of that class for My Program. 

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Wouldn't it be kind of interesting to actually have multiple classes. Because in 

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the days of yore, we talked about a Java program as really just comprised of 

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multiple classes. 

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So today's the day where you learn how to write other classes other than just 

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the main program that you're actually writing. 

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So what's the general form for actually writing a class. 

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The idea is we say "public." For right now all of our classes are gonna be public. 

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The word class, and then we give the name of the class. So this should be 

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familiar to you, just like when we wrote programs that extended Console Program, 

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we had some name here, I'll put an underline to indicate that that's not actually a literal 

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name you just give it a name, 

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and then potentially you could also add onto this, 

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"extends," 

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and then some super class. 

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And so what you're saying is that you're creating a new class with this name 

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that potentially extends some other class. And then inside you have some 

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open brace, 

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you have the body inside here, so this could be all your methods. It could also 

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be variables that are defined, as we talked about before, various instance variables or 

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ivar's in the class, 

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and then the close of the class. Now, 

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this part over here, I'll put sort of in dashed parens because it's optional. You 

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actually don't have to extend something as part of a class. 

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If you don't extend something as part of a class, 

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what Java sort of says is the default is everything in my universe, 

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at least for the time being, 

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are classes. 

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So if you don't tell me that you're extending some existing class, 

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there are some default class called "object," 

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which by default you will extend if you don't say extend some other existing 

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class. So everything at the end of the day in Java's world 

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sort of moves up the chain, or the hierarchy that we talked about a couple 

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weeks ago, and ends up an object. So everything at the end of the day really 

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is an object, and if you're not extending something in particular then you're just 

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directly an object. 

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So if you want to write this class, what you end up doing is you want to create a new 

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file. And I'll show you how to do that in Eclipse in just a second. 

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But, basically, 

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that's why I put it in the box here, the name of the file should be 

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- we kind of define it to be whatever the class name was 

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and then a .java at the end. And the .java let's us know that this is a Java program. 

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So if this thing over here, 

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this class, 

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happens to be class 

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Bob, 

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we would have some file Bob.java 

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that would be where all the code that we write for the class Bob actually lives. 

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And the stuff that's gonna be inside here is all the stuff like 

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the methods, 

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and variables, and constants, which are just a particular form of variable, they're just 

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a final variable so they get no other value. 

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And the stuff that you have inside here, the methods and the variables, 

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have certain visibility associated with them, as we talked about before. 

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These things can either be public or they can be private. 

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And the difference between those two, hopefully it'll make a little bit more sense, 

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is 

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methods or variables that are public 

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are what we refer to as being "exported out of the class." 

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So if you write a whole bunch of classes, let's say we write three 

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classes. And so I have my class Bob, 

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and Bob might have in it some public method. 

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Well, if I have some other class Mary, 

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Mary can actually call 

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the public methods of Bob. 

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So if it's public anyone can access them. 

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If they're private, they can only be accessed by other methods 

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inside of the class. 

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So if I have some method inside Bob, or some variable inside Bob that's private, 

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the only things that can refer to it are other methods inside Bob. 

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If I have some other class Mary, the methods in Mary cannot refer to the 

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private members, or the private elements of Bob. 

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So that's the important thing to keep in mind. 

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There's a couple other notions. There's a notion known as "protected." And we'll talk 

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about that potentially toward the end of class. For right now, you don't need to 

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worry about it. 

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The thing you do want to keep in mind 

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is that most things in classes will actually be private 

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unless there's a good reason to make them public. 

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In some sense think about yourself, you want to have some notion of 

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privacy. 

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You don't want to just broadcast to the world or let the world be able to come 

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in, and say, "Hey, you have some variable here which is, like, 

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your age," and someone else is gonna go and modify your age. That would kind 

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of bother you, right? And 

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so most things you keep private unless there is a good reason 

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to actually make them public. And so the run method that we 

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talked about so far, I actually sort of told you, it needs to be public. 

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Most things will be private. And I'll show you some examples of classes where we 

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look at things that are private versus public. Is 

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there any question about that, what public or private actually means or sort of 

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the general notion of how we would define the class? Hopefully, that's familiar to you 

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because you've seen it before. Um hm. [Inaudible.] 

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Right. There is someone else, 

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us, the 106a people, 

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who provided, actually the ACM people who provided the libraries for you to use, 

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that call your run method automatically when a program starts. 

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So that's what's actually going on. And when we get toward the end of the class I'll 

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kind of lift the covers on what's going on there. Um hm. [Inaudible] any class, like, that somebody's using [inaudible]? 

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Yeah, exactly. And unless you want people in Timbuktu touching stuff 

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they shouldn't be touching it should be private. 

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The way I like to think about it is think about your private parts, 

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you like to keep them private, you donft want them to be public. Most 

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things that you have on your body are private. You are an object, so most things in 

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an object are private. All right. 

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Just keep that in mind. 

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So, with that said, let's actually go and create a "new class." It's kind of fun to 

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create classes. So I want to create a new class. So what I'm gonna do is I'm 

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gonna 

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get my Eclipse out and I'm gonna fire it up. 

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And the way you can actually create a new class in Eclipse is over here in this 

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little window, that we haven't done all that much stuff with so far, 

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usually we give you some folder that's called Assignment 1 or Assignment 

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2, and you work on it. If you right click on the name of that folder, 

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and if you happen to be a Mac person that's the same thing as "control clicking" on that, 

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you'll get this big scary menu that comes up, and you just want to pick New. 

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And when you pick New you pick Class. So that's all that's going on. A right click 

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on the name of the folder, 

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pick New and pick Class. 

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And what happens now is this thing comes up, and it says, "Create a new job of class," 

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and there's all these fields, and things like dogs and cats sleeping 

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together, it's just totally out of control, 

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and the only thing you need to worry about here is what are you gonna name 

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your class. 

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And so maybe we want to write some class, 

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I'll just call this "my counter.h Because I'm gonna write a little class that creates counter. And 

235
00:11:58,210 --> 00:12:00,590
notice when I started typing, as soon as I started typing, let me get rid of this, I'll start 

236
00:12:00,590 --> 00:12:04,629
typing again "my counter." Anyone see what happened? 

237
00:12:04,629 --> 00:12:08,250
Eclipse had a cow, and the cow that it had was up here up at the top. It says, "The use 

238
00:12:08,250 --> 00:12:11,600
of the default package is discouraged." And you sit there and you think, 

239
00:12:11,600 --> 00:12:16,190
1.) What is the default package? And 2.) Why is its use discouraged? 

240
00:12:16,190 --> 00:12:19,020
And the important thing to keep in mind is you don't care. This is one of those moments in 

241
00:12:19,019 --> 00:12:21,819
your life where you're gonna be rebellious, and you're like, "Bring it on, I'm 

242
00:12:21,820 --> 00:12:24,800
in the default package," that's life in the city. 

243
00:12:24,799 --> 00:12:28,639
And what the package really is, is the package is just a collection of classes; 

244
00:12:28,639 --> 00:12:32,549
a set of classes that make sense together. Java provides a facility to say 

245
00:12:32,549 --> 00:12:34,659
we're gonna put them all in the same package. 

246
00:12:34,659 --> 00:12:37,048
Right now we're not gonna define a whole bunch of classes that all work 

247
00:12:37,048 --> 00:12:39,169
together, so we don't need to worry about packages. 

248
00:12:39,169 --> 00:12:42,519
So all of our classes that we write are gonna be in the default package, which 

249
00:12:42,519 --> 00:12:45,299
means, kind of, the un-named package, it's just sort of the package 

250
00:12:45,299 --> 00:12:48,750
that everyone's a part of unless they're a part of some other package. 

251
00:12:48,750 --> 00:12:52,080
So it says, "Oh, it's discouraged," and you're like, "I don't care I'm in the default package, 

252
00:12:52,080 --> 00:12:54,350
I'm living for big gustos." 

253
00:12:54,350 --> 00:12:59,470
And then you do something extremely complicated, which is you click "Finish." 

254
00:12:59,470 --> 00:13:00,329
And guess what? You 

255
00:13:00,328 --> 00:13:02,569
just created a new class. 

256
00:13:02,570 --> 00:13:05,290
Now, a couple of things to keep in mind: Over here, 

257
00:13:05,289 --> 00:13:08,839
in your little project that you have, there is now an entry called, "my 

258
00:13:08,840 --> 00:13:11,739
counter.java." Remember, we just named the class "counter." 

259
00:13:11,739 --> 00:13:13,240
Guess what Eclipse did for you, 

260
00:13:13,240 --> 00:13:14,568
it did this, 

261
00:13:14,568 --> 00:13:17,219
it created for you a file, 

262
00:13:17,220 --> 00:13:21,189
automatically ending with .java, that's name matched the name of the class. 

263
00:13:21,188 --> 00:13:22,349


264
00:13:22,350 --> 00:13:25,659
What else did it do for you? It created what we refer to as a "stub." 

265
00:13:25,659 --> 00:13:28,689
It said, "I'm gonna create an empty class for you." You're gonna create a class 

266
00:13:28,690 --> 00:13:31,080
called, "my counter." So what I'm gonna do is create, for you, 

267
00:13:31,080 --> 00:13:34,959
the empty shell. Right. It gives you basically two lines of code and one of them is a brace. 

268
00:13:34,958 --> 00:13:39,500
So it's not providing a whole lot to you, but it's, like, ooh, ooh, you've gotta define a class, like, 

269
00:13:39,500 --> 00:13:41,860
aren't we excited to write a class. And you're like, all right, 

270
00:13:41,860 --> 00:13:43,519
let's actually write the class. 

271
00:13:43,519 --> 00:13:47,088
So what I want to do is let's go ahead and actually write a class. 

272
00:13:47,089 --> 00:13:49,919
And the class that we're gonna write is something that's basically a 

273
00:13:49,918 --> 00:13:52,199
counter, or I'll refer to also as an "incrementor." 

274
00:13:52,200 --> 00:13:56,040
It's a very simple idea here. What I'm going to create is a class 

275
00:13:56,039 --> 00:13:58,389
that is going to allow me to get, 

276
00:13:58,389 --> 00:13:58,990
essentially, 

277
00:13:58,990 --> 00:14:02,419
numeric values that are sequentially coming from each other. You might want to say, "Why 

278
00:14:02,419 --> 00:14:05,049
would I want to do this?" This is the kind of thing, say, that Stanford would 

279
00:14:05,049 --> 00:14:07,599
actually want to do when they're assigning you ID numbers. Right? 

280
00:14:07,600 --> 00:14:10,930
They want to have some object somewhere, which is the ID number provider. And they're, 

281
00:14:10,929 --> 00:14:14,229
like, "Hey, you just came to Stanford, get me a new ID number." 

282
00:14:14,230 --> 00:14:17,048
And you just came to Stanford, "Get me the next ID number." 

283
00:14:17,048 --> 00:14:20,528
So what I want to have is some way of saying, "Hey, create this object for me that 

284
00:14:20,528 --> 00:14:23,889
just gives me numbers, and I'll just ask you for the next number," and you just 

285
00:14:23,889 --> 00:14:25,089
sequentially give me new numbers. 

286
00:14:25,089 --> 00:14:28,050
Or how many of you have actually been at a bakery or something where they have one 

287
00:14:28,049 --> 00:14:31,469
of those big red wheels that has the little tags that you pull out of it? 

288
00:14:31,470 --> 00:14:32,570
That's a 

289
00:14:32,570 --> 00:14:35,660
counter. So that's what we're gonna write as a class, and then we can create 

290
00:14:35,659 --> 00:14:37,669
objects of this counter type. Okay. 

291
00:14:37,669 --> 00:14:39,779
So how do we actually create the class? 

292
00:14:39,779 --> 00:14:43,199
This is a little bit different than writing a class that extends a program. 

293
00:14:43,200 --> 00:14:46,790
Because we're not writing a full program we're writing a class. So we don't have a run 

294
00:14:46,789 --> 00:14:47,919
method here. 

295
00:14:47,919 --> 00:14:50,528
What we do have is something that's called a "constructor." 

296
00:14:50,528 --> 00:14:53,860
And what a constructor is, is it's sort of the 

297
00:14:53,860 --> 00:14:57,370
method that initializes everything that should be going on with 

298
00:14:57,370 --> 00:14:58,528
this class. Now, here 

299
00:14:58,528 --> 00:15:01,188
is the properties of the constructor. First of all, we're gonna 

300
00:15:01,188 --> 00:15:04,328
make it public. At least for right now, all the constructors you're gonna see are gonna be 

301
00:15:04,328 --> 00:15:04,870
public. 

302
00:15:04,870 --> 00:15:07,909
A constructor does not return a value. 

303
00:15:07,909 --> 00:15:10,608
So we just skip the whole notion of a return type, 

304
00:15:10,609 --> 00:15:14,709
and we give the name of the constructor the same name as the name of the class. 

305
00:15:14,708 --> 00:15:15,929
So we say "public 

306
00:15:15,929 --> 00:15:18,639
my counter." 

307
00:15:18,639 --> 00:15:23,019
And now, at this point, we have no return type. Then we provide whatever parameters 

308
00:15:23,019 --> 00:15:24,519
we're gonna take in here. 

309
00:15:24,519 --> 00:15:27,429
Now, what I'm gonna do for my little counter is I'm gonna be kind of funky, 

310
00:15:27,429 --> 00:15:29,979
I'm not necessarily gonna start at one, I'm gonna allow someone to say, 

311
00:15:29,980 --> 00:15:34,028
"Hey, you know, like, Stanford ID's don't start at one, like, they actually start at, like, 30-some 

312
00:15:34,028 --> 00:15:36,909
odd million." Right. And most of you actually have 

313
00:15:36,909 --> 00:15:39,308
one that's in, like, the 40-odd millions 

314
00:15:39,308 --> 00:15:41,838
or probably 50-odd millions. Is your first ID No. a five now? 

315
00:15:41,839 --> 00:15:44,590
How many people have a five? Anyone have a six? Anyone 

316
00:15:44,590 --> 00:15:46,700
have a four? 

317
00:15:46,700 --> 00:15:50,030
Anyone have a three? Yeah, 

318
00:15:50,029 --> 00:15:53,120
good times. It's come a long way, evidently. We've admitted 20 million 

319
00:15:53,120 --> 00:15:55,100
students in the last 15 years. 

320
00:15:55,100 --> 00:15:59,000
So what we're gonna allow my counter to do is have some starting values. So we're 

321
00:15:59,000 --> 00:16:01,599
gonna specify parameter start value 

322
00:16:01,599 --> 00:16:05,160
that's going to be, essentially, where we want this counter to start. 

323
00:16:05,159 --> 00:16:08,519
And now you think for a moment, you think, "Hey, we've gotta do something with that 

324
00:16:08,519 --> 00:16:09,688
start value," right? 

325
00:16:09,688 --> 00:16:13,498
This is state of the object that we need to keep around. Somehow we need to start 

326
00:16:13,499 --> 00:16:16,139
off by saying this counter starts at start value, and 

327
00:16:16,139 --> 00:16:19,068
every time you ask me for a new value of the counter I'm gonna give you a new 

328
00:16:19,068 --> 00:16:19,990
number. 

329
00:16:19,990 --> 00:16:24,289
That means I need to keep track between method calls of what the old number was. 

330
00:16:24,289 --> 00:16:24,879


331
00:16:24,879 --> 00:16:27,710
So if I need to keep track of something between method calls what am I gonna 

332
00:16:27,710 --> 00:16:30,370
need? 

333
00:16:30,370 --> 00:16:34,120
Instance variable. There's a few people over here, 

334
00:16:34,120 --> 00:16:36,509
one person lying on the floor down there. 

335
00:16:36,509 --> 00:16:39,939
So what I'm gonna have is an instance variable. And to find the instance variable 

336
00:16:39,940 --> 00:16:43,020
down at the bottom I'm going to make the instance variable private because I 

337
00:16:43,019 --> 00:16:46,139
don't want someone being able to muck with my instance variable. So I'm going 

338
00:16:46,139 --> 00:16:48,309
to have private int, 

339
00:16:48,309 --> 00:16:52,789
and I'll just call this "counter." So this is my instance variable over here. And 

340
00:16:52,789 --> 00:16:55,379
so what I'm going to do is, in my constructor 

341
00:16:55,379 --> 00:16:57,239
I'm going to say, "Hey, 

342
00:16:57,239 --> 00:17:01,550
initialize that counter to be equal to whatever start value the user gave me." 

343
00:17:01,549 --> 00:17:05,328
So I have my own little instance variable here. Every counter object is 

344
00:17:05,328 --> 00:17:07,349
gonna have its own little counter variable, 

345
00:17:07,349 --> 00:17:10,538
and I'm just gonna start it off by giving it whatever value the user told 

346
00:17:10,538 --> 00:17:13,939
me to set it to when they created an object of this type. And I'll show you in just a 

347
00:17:13,939 --> 00:17:16,199
second how we create an object of this type. 

348
00:17:16,199 --> 00:17:19,240
Now, the other thing we could do, that's kind of funky, is you can actually have more than 

349
00:17:19,240 --> 00:17:20,750
one constructor. 

350
00:17:20,750 --> 00:17:24,000
You can have multiple constructors that have the same 

351
00:17:24,000 --> 00:17:28,068
form, they're all public, at least so far. They return no type. 

352
00:17:28,068 --> 00:17:31,418
But the thing that differentiates them, and the way the computer knows which 

353
00:17:31,419 --> 00:17:32,528
constructor to actually use 

354
00:17:32,528 --> 00:17:35,940
is what the parameters are. So we just created a constructor that has one parameter. 

355
00:17:35,940 --> 00:17:38,940
I'm also going to create another constructor that has no parameters. 

356
00:17:38,940 --> 00:17:42,049
So someone can actually create a counter without telling me what the starting value 

357
00:17:42,049 --> 00:17:45,299
is. Well, what is the starting value if they don't give me one? I still need to 

358
00:17:45,299 --> 00:17:45,819
assign one. 

359
00:17:45,819 --> 00:17:48,539
So I'm gonna say, "Hey, if you didn't give me one, I'm gonna set 

360
00:17:48,539 --> 00:17:49,490
the 

361
00:17:49,490 --> 00:17:52,460
starting value to be one." Because most of the time when people want a 

362
00:17:52,460 --> 00:17:53,808
counter they just want to count from one. 

363
00:17:53,808 --> 00:17:57,028
But if you want to do something funky, like have ID numbers for a university, 

364
00:17:57,028 --> 00:17:58,819
you can tell me the starting value. 

365
00:17:58,819 --> 00:18:02,109
And so I'll show you in just a second, how we actually create objects of this type, 

366
00:18:02,109 --> 00:18:05,038
and when we create objects how it differentiates between whether to call 

367
00:18:05,038 --> 00:18:10,440
this method up here with a parameter or this method over here without a parameter. A 

368
00:18:10,440 --> 00:18:14,179
couple of other things we need to do. We need to find and specify some way of saying, 

369
00:18:14,179 --> 00:18:18,150
"Give me the next value." Right. How do I get new numbers, how do I request new 

370
00:18:18,150 --> 00:18:20,870
numbers from this counter? Well, I need to have some method. 

371
00:18:20,869 --> 00:18:24,359
This method's going to return for me an integer, because my counter is just 

372
00:18:24,359 --> 00:18:24,949
integers, 

373
00:18:24,950 --> 00:18:27,080
and I'll call this "next value." 

374
00:18:27,079 --> 00:18:30,589
It takes in no parameters because I don't need to tell you anything. I don't need to tell 

375
00:18:30,589 --> 00:18:33,538
the object anything to get the next value. I just say, "Hey, buddy, 

376
00:18:33,538 --> 00:18:37,538
next value." And it gives me the next value. So the way it's gonna give me the next 

377
00:18:37,538 --> 00:18:41,890
value is it's going to give me whatever value the counter has right now. 

378
00:18:41,890 --> 00:18:44,590
So I might be inclined to say, "return counter." 

379
00:18:44,589 --> 00:18:49,419
What's the problem with doing that? 

380
00:18:49,420 --> 00:18:52,700
What happens the next time I call next value? 

381
00:18:52,700 --> 00:18:55,769
I return the same. But that's not very exciting, right. You go into the bakery, and you're like, 

382
00:18:55,769 --> 00:18:59,440
"Hey, I got No. 1." And you look around and there's 300 other people that have No. 1. The 

383
00:18:59,440 --> 00:19:01,058
fights break out, it's bad times. 

384
00:19:01,058 --> 00:19:03,960
So what we need to do is we need to say, "Hey, 

385
00:19:03,960 --> 00:19:07,480
I need to keep track of what value the counter is currently right now because that's what I'm gonna return to you." So I'm gonna have some 

386
00:19:07,480 --> 00:19:10,470
"temporary." 

387
00:19:10,470 --> 00:19:15,000
And this temporary is a local variable, it only needs to live inside this method 

388
00:19:15,000 --> 00:19:16,779
for the lifetime of this method. 

389
00:19:16,779 --> 00:19:21,279
And I will initialize that temporary to be whatever the current counter value is. 

390
00:19:21,279 --> 00:19:24,849
Then I'll go ahead and add one to the counter value. So I'll increment the counter 

391
00:19:24,849 --> 00:19:28,039
so the number next time this gets called is gonna be one higher. 

392
00:19:28,039 --> 00:19:31,170
And what am I gonna do now when I return? 

393
00:19:31,170 --> 00:19:33,170
Any ideas? 

394
00:19:33,170 --> 00:19:36,340
Return the temporary. The temporary 

395
00:19:36,339 --> 00:19:38,169
is 

396
00:19:38,170 --> 00:19:41,440
what the value of the counter started at when this method was first called. So I said, 

397
00:19:41,440 --> 00:19:43,409
"Oh, store off a copy of it, 

398
00:19:43,409 --> 00:19:47,200
add one to the counter," because next time it'll be one greater and return whatever the old 

399
00:19:47,200 --> 00:19:48,278
value was in temporary. 

400
00:19:48,278 --> 00:19:51,669
And after this function's gone, temporary gets blown away. And it's like, "Hey, I was a 

401
00:19:51,669 --> 00:19:54,830
local variable, I gave my life for the method." And we're like, "Thank you very much, 

402
00:19:54,829 --> 00:19:55,909
temporary, that was 

403
00:19:55,910 --> 00:19:56,929
very 

404
00:19:56,929 --> 00:20:01,038
courageous of you." But you're gone now but you managed to return this 

405
00:20:01,038 --> 00:20:05,009
value that you stored. So good times, you actually did what we needed you to do. 

406
00:20:05,009 --> 00:20:08,680
And this is my whole class. Is there 

407
00:20:08,680 --> 00:20:12,929
any questions about this class? Now, 

408
00:20:12,929 --> 00:20:15,800
let me show you one other thing before we actually 

409
00:20:15,799 --> 00:20:17,950
make use of this class. 

410
00:20:17,950 --> 00:20:19,048
And it's 

411
00:20:19,048 --> 00:20:21,970
a way that we could have actually done things slightly differently. 

412
00:20:21,970 --> 00:20:24,990
So when we actually created the class here's our constructor. 

413
00:20:24,990 --> 00:20:28,839
In this case, what I've done is I've just renamed the class from my counter to be 

414
00:20:28,839 --> 00:20:31,859
incrementor, because incrementor is kind of the more funky computer science term 

415
00:20:31,859 --> 00:20:33,669
for it, and incrementor is something that 

416
00:20:33,670 --> 00:20:35,419
adds one to something and counts. 

417
00:20:35,419 --> 00:20:39,020
So other than my counter, which is just all fuzzy and good, it's like, "Oh, it's My Counter," 

418
00:20:39,019 --> 00:20:41,889
but people might think, "Oh, your counter, is that, like, the 

419
00:20:41,890 --> 00:20:44,230
restaurant in Palo Alto that gives you burgers?" 

420
00:20:44,230 --> 00:20:45,960
Anyone ever eaten there? It's 

421
00:20:45,960 --> 00:20:48,049
a good time. 

422
00:20:48,049 --> 00:20:51,149
And then they didn't even pay for the product placement, I've gotta say. 

423
00:20:51,150 --> 00:20:54,310
We'll call it incrementor. But same kind of thing here, we're gonna have 

424
00:20:54,309 --> 00:20:57,960
some constructor for incrementor. It's gonna take some start value. I'm just showing 

425
00:20:57,960 --> 00:20:59,680
one of the constructors here. 

426
00:20:59,680 --> 00:21:02,210
And one thing you might say is, 

427
00:21:02,210 --> 00:21:05,190
what are the properties, just to reiterate, of constructors? The name 

428
00:21:05,190 --> 00:21:08,179
of the class is the same as the constructor name. The constructor does 

429
00:21:08,179 --> 00:21:12,528
not specify return type, it's responsible for initializing whatever values we care 

430
00:21:12,528 --> 00:21:17,089
about in the object. And this puppy gets called when an object is created. And 

431
00:21:17,089 --> 00:21:20,730
I'll show you an example. Just like you created graphical objects in the past, 

432
00:21:20,730 --> 00:21:23,929
we're gonna create an object an object of type incrementor in just a second, and I'll show 

433
00:21:23,929 --> 00:21:24,989
the code for that. 

434
00:21:24,989 --> 00:21:29,660
But that's when this implementor method actually gets invoked. 

435
00:21:29,660 --> 00:21:32,009
So one thing that people think about when they see this is, 

436
00:21:32,009 --> 00:21:35,240
"Hey, you have this start value thing, and then over here you call it counter, why 

437
00:21:35,240 --> 00:21:37,940
don't you just call it counter both places? 

438
00:21:37,940 --> 00:21:41,519
Why don't you rewrite the code like that?" 

439
00:21:41,519 --> 00:21:47,470
What happens when we do that? Is there any issue with doing that? You're, like, ah. 

440
00:21:47,470 --> 00:21:50,970
There's the one part of you that's, like, "Oh, that looks so clean," like, counter equals counter. And then there's a part of 

441
00:21:50,970 --> 00:21:53,299
you that get really uneasy. 

442
00:21:53,299 --> 00:21:55,009
Because you see this line here? 

443
00:21:55,009 --> 00:21:56,569
That line does nothing. 

444
00:21:56,569 --> 00:22:00,460
It assigns the same value back to itself. And you're, like, "But I wanted this 

445
00:22:00,460 --> 00:22:03,680
counter to be like my instance variable counter, and I wanted this counter 

446
00:22:03,680 --> 00:22:05,620
to be my parameter counter, can't the 

447
00:22:05,619 --> 00:22:07,919
computer just figure that out?" 

448
00:22:07,920 --> 00:22:09,130


449
00:22:09,130 --> 00:22:11,700
No. 

450
00:22:11,700 --> 00:22:14,590
You can't do that here 

451
00:22:14,589 --> 00:22:18,399
because sometimes it 

452
00:22:18,400 --> 00:22:20,380
will prevent you. So the problem here 

453
00:22:20,380 --> 00:22:21,580
is that the 

454
00:22:21,579 --> 00:22:24,210
parameter counter is actually getting assigned to itself. 

455
00:22:24,210 --> 00:22:28,000
You're, like, "Hey, the counter here before was the instance variable, what's going on?" 

456
00:22:28,000 --> 00:22:31,460
If you have a parameter that has the same name as one of your instance 

457
00:22:31,460 --> 00:22:34,090
variables, you get something called, "shadowing." 

458
00:22:34,089 --> 00:22:37,980
What shadowing is, is when I refer to some variable, the first place I look is 

459
00:22:37,980 --> 00:22:38,489
do I 

460
00:22:38,489 --> 00:22:42,140
have any parameters or local variables that have that name. 

461
00:22:42,140 --> 00:22:47,220
If I do then that's what I'm referring to. If I don't have any parameters or local 

462
00:22:47,220 --> 00:22:50,640
variables with that name then I check to see if I have an instance variable with that 

463
00:22:50,640 --> 00:22:54,320
name. So there's actually an order in which I check for these names. And because in 

464
00:22:54,319 --> 00:22:58,359
this case I have a parameter named counter, this counter's referring to that parameter, 

465
00:22:58,359 --> 00:23:00,639
and this counter's referring to that parameter. 

466
00:23:00,640 --> 00:23:02,778
In the previous case, 

467
00:23:02,778 --> 00:23:06,190
I didn't have a parameter named counter. So it said, "Do I have a parameter named counter?" 

468
00:23:06,190 --> 00:23:09,870
"No." "Do I have an instance variable named counter?" "Yes." So it uses the instance 

469
00:23:09,869 --> 00:23:10,779
variable. 

470
00:23:10,779 --> 00:23:14,740
So you could say, "Hey, is there a way I can force it to know the difference between the 

471
00:23:14,740 --> 00:23:17,259
two counters?" And the answer is actually yes. 

472
00:23:17,259 --> 00:23:19,150
So even though there's a problem here 

473
00:23:19,150 --> 00:23:20,970
we can fix this. 

474
00:23:20,970 --> 00:23:24,470
And the way we fix this is with a notion called, "this." 

475
00:23:24,470 --> 00:23:26,569
The way you can think of this, 

476
00:23:26,569 --> 00:23:31,689
is this is always referring to the receiving object. What does that mean? 

477
00:23:31,690 --> 00:23:32,950
That's a mouthful. 

478
00:23:32,950 --> 00:23:36,480
It means that when some method gets called, remember when method gets called 

479
00:23:36,480 --> 00:23:39,569
there's always some object that that method is getting called on, 

480
00:23:39,569 --> 00:23:42,079
well, when we call that method, 

481
00:23:42,079 --> 00:23:45,819
inside the method we can refer to this.counter, which says 

482
00:23:45,819 --> 00:23:50,679
refer to myself, my own object counter, don't refer to my parameter 

483
00:23:50,680 --> 00:23:54,619
counter. So this.counter will look for an instance variable directly. 

484
00:23:54,618 --> 00:23:58,579
It will not look for a local variable; it will not look for a parameter. 

485
00:23:58,579 --> 00:24:02,250
So here when I say, this.counter, it says you have an instance variable named 

486
00:24:02,250 --> 00:24:05,089
counter. That's what this.counter is referring to. This 

487
00:24:05,089 --> 00:24:07,059
counter over here, it says, 

488
00:24:07,059 --> 00:24:09,379
"Hey, I don't have any kind of qualifier." 

489
00:24:09,380 --> 00:24:12,530
Let me check the parameters and local variables first. I have a parameter named 

490
00:24:12,529 --> 00:24:15,779
counter, so that's what the counter over here is actually referring to. 

491
00:24:15,779 --> 00:24:19,538
Now, this is, kind of, ugly syntax. There's some reasons to use it here and there, but 

492
00:24:19,538 --> 00:24:22,490
it's ugly syntax. The way to think about this is 

493
00:24:22,490 --> 00:24:25,660
just give them different names, that's the easiest way to do it. 

494
00:24:25,660 --> 00:24:28,670
Keep the names of your parameters distinct from the name of your instance 

495
00:24:28,670 --> 00:24:31,720
variables and there'll just be no confusion with this or that or whatever, you just 

496
00:24:31,720 --> 00:24:35,809
keep the names distinct. And it's the preferred root. You want to think about writing 

497
00:24:35,809 --> 00:24:37,288
programs that require the human 

498
00:24:37,288 --> 00:24:40,538
to think less when they're actually looking at the program. You're, like, "That's 

499
00:24:40,538 --> 00:24:42,878
really weird, I thought computer science was all about thinking." 

500
00:24:42,878 --> 00:24:45,949
Yeah, it's about thinking higher up the food chain, you don't want to worry about 

501
00:24:45,950 --> 00:24:49,889
little details like this and get caught in these weird shadowing effects or aliasing 

502
00:24:49,888 --> 00:24:53,969
effects, you just want to say, "Yeah, they're distinct," and then it's clear. Is there are 

503
00:24:53,970 --> 00:24:56,049
any questions about that? It's kind of 

504
00:24:56,049 --> 00:24:58,930
a funky concept but it's just important for you to see it. 

505
00:24:58,930 --> 00:25:01,870
So with that said, let's actually look at 

506
00:25:01,869 --> 00:25:05,519
not my counter anymore. We sort of wrote the bare bones code for my counter, 

507
00:25:05,519 --> 00:25:08,700
but here are the full-fledged incrementor version. And so when we're now 

508
00:25:08,700 --> 00:25:11,000
computer scientists, and we think about good programming style, 

509
00:25:11,000 --> 00:25:14,190
we have some comment for the class. So just like the programs you wrote, you want to 

510
00:25:14,190 --> 00:25:17,769
have some comment for the whole class, you want to have some comment per method 

511
00:25:17,769 --> 00:25:22,339
you write. So this is all the same code we just wrote, now it's just commented. 

512
00:25:22,339 --> 00:25:29,339
So good programming style. Um hm. [Inaudible.] 

513
00:25:29,819 --> 00:25:31,730
Because if I declare it every time 

514
00:25:31,730 --> 00:25:35,089
there is no way of me being able to keep track of what it's previous value was. 

515
00:25:35,089 --> 00:25:38,189
So that's the critical concept of an instance variable, I need to 

516
00:25:38,190 --> 00:25:42,679
keep track of some information between method calls. If I didn't, like temp, I 

517
00:25:42,679 --> 00:25:46,570
didn't care what temp was the last time I called next value, it's just the local 

518
00:25:46,569 --> 00:25:49,299
variable and it goes away. But counter I need to keep track of. 

519
00:25:49,299 --> 00:25:51,190
So how do I actually use counter? 

520
00:25:51,190 --> 00:25:54,038
I wrote a program called "use 

521
00:25:54,038 --> 00:25:56,869
counter." 

522
00:25:56,869 --> 00:25:58,869
So 

523
00:25:58,869 --> 00:26:02,669
use counter is basically just a Java program that is a program. 

524
00:26:02,670 --> 00:26:05,509
So it extends Console Program, just like you're used to. 

525
00:26:05,509 --> 00:26:08,410
I'm changing the font. You donft need to worry about this line. All this line does is 

526
00:26:08,410 --> 00:26:11,710
it just makes the font bigger so when I run the program you can actually see the text. 

527
00:26:11,710 --> 00:26:15,069
Because there were people who were having trouble seeing the output text when I ran 

528
00:26:15,069 --> 00:26:16,329
programs. Now, 

529
00:26:16,329 --> 00:26:20,308
how do I create objects of this class? This is just like you saw with graphical 

530
00:26:20,308 --> 00:26:22,990
objects. 

531
00:26:22,990 --> 00:26:26,039
What's going on is if I want to create some new counter, 

532
00:26:26,039 --> 00:26:27,720
or some new incrementor, 

533
00:26:27,720 --> 00:26:31,329
I specify the type of the variable incrementor. I give it a name, I'll call it 

534
00:26:31,329 --> 00:26:32,379
"count one." 

535
00:26:32,380 --> 00:26:36,130
And because I want to create an object, I go to the incrementor factory and 

536
00:26:36,130 --> 00:26:36,610
say, 

537
00:26:36,609 --> 00:26:39,369
"Give me a new incrementor." 

538
00:26:39,369 --> 00:26:43,729
When I say new and the name of a class, what I get is a new object of that class. 

539
00:26:43,730 --> 00:26:45,929
So at this point when I say new incrementor, 

540
00:26:45,929 --> 00:26:48,960
a new incrementor object is created for me. 

541
00:26:48,960 --> 00:26:52,789
That is when the constructor for that object gets invoked. So that constructor 

542
00:26:52,789 --> 00:26:56,779
thing we just wrote, that initializes the object, the place it gets called is when 

543
00:26:56,779 --> 00:27:00,470
you ask for a new one of the class, when you get a new object the constructor's 

544
00:27:00,470 --> 00:27:01,230
invoked. 

545
00:27:01,230 --> 00:27:04,839
Which version of the constructor does it invoke if you have multiple versions? 

546
00:27:04,839 --> 00:27:08,168
It depends on the parameters you give. Right. We created two versions, one 

547
00:27:08,169 --> 00:27:09,429
without a parameter, 

548
00:27:09,429 --> 00:27:13,070
if you didn't specify the parameter it set the counter starting at one. One 

549
00:27:13,069 --> 00:27:16,298
with a parameter, so if you specify parameter, it says, "Hey, there was a version 

550
00:27:16,298 --> 00:27:19,808
of this that had a parameter," that's the version that's gonna get invoked, and so 

551
00:27:19,808 --> 00:27:22,779
it's gonna set the beginning counter to 

552
00:27:22,779 --> 00:27:26,059
1,000. Any questions about that? So what are 

553
00:27:26,059 --> 00:27:29,500
we gonna do? Now what we're gonna do is write out some values of the counter and count up. So 

554
00:27:29,500 --> 00:27:30,489
what I'm gonna do 

555
00:27:30,489 --> 00:27:31,798
is write a method. 

556
00:27:31,798 --> 00:27:35,359
This method is going to take an object as a parameter. There's no reason why it 

557
00:27:35,359 --> 00:27:38,240
can't, parameters just have some type and some name. 

558
00:27:38,240 --> 00:27:41,900
So this count five times method is not gonna return anything, but it's going 

559
00:27:41,900 --> 00:27:44,030
to take as a parameter 

560
00:27:44,029 --> 00:27:47,500
something of type incrementor, that I'll just call "counter." And what it's gonna 

561
00:27:47,500 --> 00:27:52,039
do is very simple, it's going to loop through five times and just print out 

562
00:27:52,039 --> 00:27:56,019
the next five values of that counter. So that's all it does, it just pulls the 

563
00:27:56,019 --> 00:27:59,289
next five tickets from the counter and writes them out to the screen. 

564
00:27:59,289 --> 00:28:03,019
So I'm gonna say, what are the first five values for count one, and call 

565
00:28:03,019 --> 00:28:06,480
this function. What are the first five values for count two, and call the 

566
00:28:06,480 --> 00:28:10,579
function passing and count two, and what are another five values for 

567
00:28:10,579 --> 00:28:13,710
count one. So let me go ahead and run this program and you can see what's going on. 

568
00:28:13,710 --> 00:28:15,180
So if I run this, 

569
00:28:15,180 --> 00:28:20,640


570
00:28:20,640 --> 00:28:22,620


571
00:28:22,619 --> 00:28:23,839
there's what I get. 

572
00:28:23,839 --> 00:28:27,259
So I say the first five values for count one. Remember count one 

573
00:28:27,259 --> 00:28:30,410
started with a counter of one. It gives me one, two, three, four, five. 

574
00:28:30,410 --> 00:28:33,788
Then I asked for the first five value of count two. Count two started at 1,000, so 

575
00:28:33,788 --> 00:28:36,089
I get a 1,000 up to a 1,004. 

576
00:28:36,089 --> 00:28:39,069
Then I ask for the next five values of count one, 

577
00:28:39,069 --> 00:28:41,509
and I get six, seven, eight, nine, ten. 

578
00:28:41,509 --> 00:28:44,769
Now, this all seems fairly straightforward, except for one fact that 

579
00:28:44,769 --> 00:28:47,829
should slightly disturb you. Is there anything that slightly disturbs you? 

580
00:28:47,829 --> 00:28:48,159


581
00:28:48,160 --> 00:28:51,450


582
00:28:51,450 --> 00:28:52,980


583
00:28:52,980 --> 00:28:56,349
The thing that should slightly disturb you is the conversation we had last time, 

584
00:28:56,349 --> 00:28:59,819
where we talked about when you pass a parameter to something you get a copy of 

585
00:28:59,819 --> 00:29:01,009
that parameter, 

586
00:29:01,009 --> 00:29:05,129
or you get a copy of that thing that you're passing. So 

587
00:29:05,130 --> 00:29:07,680
the interesting thing that's going on is you would think, 

588
00:29:07,680 --> 00:29:10,480
well, you're passing count one here. 

589
00:29:10,480 --> 00:29:13,660
When you pass count one there shouldn't you have gotten a copy of the count one 

590
00:29:13,660 --> 00:29:15,610
that started with its counter of one? 

591
00:29:15,609 --> 00:29:19,899
So when it counts it up here shouldn't it have only been counting in that copy, 

592
00:29:19,900 --> 00:29:23,410
and so when it was done here that copy should have gone away, and when you returned 

593
00:29:23,410 --> 00:29:26,750
over here your counter should have still been one? 

594
00:29:26,750 --> 00:29:29,839
If we were doing integers that's sort of what would have happened. 

595
00:29:29,839 --> 00:29:33,669
But the things that's different between integers and doubles and objects, and this is the 

596
00:29:33,670 --> 00:29:35,730
important thing to think about objects, 

597
00:29:35,730 --> 00:29:38,139
when you pass an object as a parameter 

598
00:29:38,138 --> 00:29:42,339
you are not passing a copy of the object, you're passing what we refer to as a reference to 

599
00:29:42,339 --> 00:29:43,199
the object, 

600
00:29:43,200 --> 00:29:44,640
which you're actually passing 

601
00:29:44,640 --> 00:29:47,080
the object itself. 

602
00:29:47,079 --> 00:29:50,839
So what we think of as a reference is really there is a way to point to the object. 

603
00:29:50,839 --> 00:29:52,918
So let's say you're an object. 

604
00:29:52,919 --> 00:29:56,179
When I'm over here and I say give me a new object, you're sort of created, you come into 

605
00:29:56,179 --> 00:29:57,070
being. 

606
00:29:57,069 --> 00:30:00,720
And what count one really is, is just a way of pointing to you. 

607
00:30:00,720 --> 00:30:04,600
And so when I say, "Hey, count five times, count one," what I'm actually passing 

608
00:30:04,599 --> 00:30:07,158
is a copy of my finger. 

609
00:30:07,159 --> 00:30:08,460
And you're, like, that's a little weird, 

610
00:30:08,460 --> 00:30:09,750
but it points to you. 

611
00:30:09,750 --> 00:30:14,019
So when we count through five times you're the one counting five times. 

612
00:30:14,019 --> 00:30:18,069
And then when we come back over here, and I call count one again, I'm passing 

613
00:30:18,069 --> 00:30:21,759
another copy of my finger that's pointing to the same place. So 

614
00:30:21,759 --> 00:30:26,250
you're the same object that was invoked before, and you give me the next five values. 

615
00:30:26,250 --> 00:30:29,440
So critical thing to remember, when you are passing an 

616
00:30:29,440 --> 00:30:31,528
object as a parameter, 

617
00:30:31,528 --> 00:30:34,690
you are actually in some sense passing the object itself. The way you can think 

618
00:30:34,690 --> 00:30:37,440
about this is it's like the mafia. 

619
00:30:37,440 --> 00:30:41,179
We know where objects live, and if I know where you live when I'm doing something, 

620
00:30:41,179 --> 00:30:44,100
when I, like, come to your house and put a horse head in your bed or I'm, like, 

621
00:30:44,099 --> 00:30:46,449
getting the next value or whatever, 

622
00:30:46,450 --> 00:30:47,350
it's you, 

623
00:30:47,349 --> 00:30:49,879
and it's you every time because I know where you live. 

624
00:30:49,880 --> 00:30:53,230
With integers, we don't know where they live, we just pass copies. Here's a copy of 

625
00:30:53,230 --> 00:30:55,989
a double, here's a copy of an integer, we don't know where they live. 

626
00:30:55,989 --> 00:30:57,259
That's perfectly fine. 

627
00:30:57,259 --> 00:31:00,519
With objects, we know where they live. And so what you're actually passing is 

628
00:31:00,519 --> 00:31:02,710
you're passing where that object lives, 

629
00:31:02,710 --> 00:31:05,929
and so that way you're always referring to the same object because you're referring to the 

630
00:31:05,929 --> 00:31:09,950
same address where that object lives, you're always looking up the object in the same place. 

631
00:31:09,950 --> 00:31:11,640
Any questions about that? 

632
00:31:11,640 --> 00:31:15,250
That's kind of a critical concept. When we write a program 

633
00:31:15,250 --> 00:31:19,200
that makes use of a class that we've written 

634
00:31:19,200 --> 00:31:23,720
how does it know to access that class? 

635
00:31:23,720 --> 00:31:24,809
Because 

636
00:31:24,809 --> 00:31:28,168
when you're creating your classes over here you're creating all your classes 

637
00:31:28,169 --> 00:31:31,110
in this default package and they're all in the same project. 

638
00:31:31,109 --> 00:31:34,428
So when you say give me a new incrementor, it goes and looks through these, and says, 

639
00:31:34,429 --> 00:31:41,429
"Hey, do you have an incrementor class over there," and that's how it actually knows. For the third part of this program, 

640
00:31:48,700 --> 00:31:51,840
[inaudible]? Because we already counted one, two, three, four, five. So the counter of the value 

641
00:31:51,839 --> 00:31:58,369
that's actually being stored for the counter 

642
00:31:58,369 --> 00:32:00,609


643
00:32:00,609 --> 00:32:03,849
is 

644
00:32:03,849 --> 00:32:08,039


645
00:32:08,039 --> 00:32:10,009


646
00:32:10,009 --> 00:32:13,759


647
00:32:13,759 --> 00:32:17,430


648
00:32:17,430 --> 00:32:20,580


649
00:32:20,579 --> 00:32:23,980


650
00:32:23,980 --> 00:32:25,940


651
00:32:25,940 --> 00:32:29,090


652
00:32:29,089 --> 00:32:32,629
currently 

653
00:32:32,630 --> 00:32:34,130
six. 

654
00:32:34,130 --> 00:32:38,330


655
00:32:38,329 --> 00:32:41,250
Any other questions? 

656
00:32:41,250 --> 00:32:44,750


657
00:32:44,750 --> 00:32:48,638
So 

658
00:32:48,638 --> 00:32:50,538


659
00:32:50,538 --> 00:32:52,990
one thing we can also do that's kind of funky 

660
00:32:52,990 --> 00:32:56,819
is, remember when we have our counter, or our incrementor, we have this instance 

661
00:32:56,819 --> 00:32:58,049
variable. 

662
00:32:58,049 --> 00:33:01,639
There's another kind of variable, which I never told you about before, well, actually, 

663
00:33:01,640 --> 00:33:04,820
I did tell you about it, I just didn't give you its actual name before, but now 

664
00:33:04,819 --> 00:33:06,460
you're not old enough 

665
00:33:06,460 --> 00:33:12,029
so I'll tell you what the real deal is. 

666
00:33:12,029 --> 00:33:13,129


667
00:33:13,130 --> 00:33:15,270


668
00:33:15,269 --> 00:33:19,240


669
00:33:19,240 --> 00:33:23,399
The basic idea is so far you've seen things like local variables, 

670
00:33:23,398 --> 00:33:26,898
and you've seen instance variables. There is a new kind of variable, 

671
00:33:26,898 --> 00:33:28,129
and it is called a 

672
00:33:28,130 --> 00:33:30,990
"class variable" or a class var. 

673
00:33:30,990 --> 00:33:35,769
What a class variable is, it is a variable that is shared by all objects 

674
00:33:35,769 --> 00:33:40,809
of that class. There is one variable that all objects of that class share, that's 

675
00:33:40,809 --> 00:33:44,069
the funky concept. In an instance variable 

676
00:33:44,069 --> 00:33:48,789
each object of that class has a different version of that variable. 

677
00:33:48,789 --> 00:33:51,398
So if we have some counter over here, and if we have 

678
00:33:51,398 --> 00:33:55,000
some count one, and some count two, and they each have 

679
00:33:55,000 --> 00:33:59,000
an instance variable that we just defined, each of them has its own box. 

680
00:33:59,000 --> 00:34:05,730
If I were to create a class variable, there is one variable counter that all 

681
00:34:05,730 --> 00:34:09,510
objects, count one, count two, all objects of 

682
00:34:09,510 --> 00:34:10,729
the type, 

683
00:34:10,728 --> 00:34:12,648
or of the class, that this 

684
00:34:12,648 --> 00:34:14,588
class variable is in, 

685
00:34:14,588 --> 00:34:19,588
are all referring to the same one single box. Now, 

686
00:34:19,588 --> 00:34:22,449
there's sometimes you want to do that. I'll just show you a very simple example of 

687
00:34:22,449 --> 00:34:25,930
how that works, and how you actually specify it. So the way you specify it, 

688
00:34:25,929 --> 00:34:29,409
is in instance variable you just specified if it was public or private, and you specified 

689
00:34:29,409 --> 00:34:31,128
its type and you gave it a name. 

690
00:34:31,128 --> 00:34:36,278
In class variable you add this word called "static." And sometimes class variables as 

691
00:34:36,278 --> 00:34:38,840
a result are referred to as static variables. And 

692
00:34:38,840 --> 00:34:42,000
you've seen static before. When did you actually see static used in conjunction 

693
00:34:42,000 --> 00:34:44,108
with a variable? 

694
00:34:44,108 --> 00:34:45,389
Constants, right. 

695
00:34:45,389 --> 00:34:48,989
Why did we make them class variables by adding static? Because it doesn't make 

696
00:34:48,989 --> 00:34:53,119
sense for two different objects that are circles to have their own copy of the 

697
00:34:53,119 --> 00:34:54,838
constant value pi. 

698
00:34:54,838 --> 00:34:57,659
Pi is in some sense some universal for all circles. 

699
00:34:57,659 --> 00:35:02,998
So it would be shared as a class variable among all circles. 

700
00:35:02,998 --> 00:35:04,649
So even though it was a constant, 

701
00:35:04,650 --> 00:35:08,338
if we didn't say static, we would have gotten one of those constant values for 

702
00:35:08,338 --> 00:35:10,429
every circle we created, for example. 

703
00:35:10,429 --> 00:35:13,339
We want to just have one that's shared, so we call it static. 

704
00:35:13,340 --> 00:35:16,420
But you can have static variables that are not necessarily constants, they don't 

705
00:35:16,420 --> 00:35:20,019
have to be final. So let me show you an example on the computer. 

706
00:35:20,018 --> 00:35:22,679
We come over here. Here's our incrementor that we just wrote. 

707
00:35:22,679 --> 00:35:24,639
And we say private 

708
00:35:24,639 --> 00:35:25,348
static 

709
00:35:25,349 --> 00:35:26,300
int counter. 

710
00:35:26,300 --> 00:35:29,440
So what I've now done is, instead of creating an instance variable I have 

711
00:35:29,440 --> 00:35:31,499
created a class variable. 

712
00:35:31,498 --> 00:35:35,209
How does this change the program? Let me save this and run this. 

713
00:35:35,210 --> 00:35:39,380
What this will now do is, all of the things that I create that are incrementors 

714
00:35:39,380 --> 00:35:39,789


715
00:35:39,789 --> 00:35:42,739
are all referring to the same counter. 

716
00:35:42,739 --> 00:35:47,728
You're, like, whoa, that's kind of funky. Yeah, let me show you what actually happens. 

717
00:35:47,728 --> 00:35:51,259
So when I run use counter, 

718
00:35:51,259 --> 00:35:53,039
I get 

719
00:35:53,039 --> 00:35:56,749
a 1,000 through a 1,004, then I get 1,005 through a 1,009, 

720
00:35:56,748 --> 00:35:59,759
then I get 1,010 through a 1,014. 

721
00:35:59,759 --> 00:36:01,458
And you're, like, whoa, 

722
00:36:01,458 --> 00:36:03,409
what happened there? 

723
00:36:03,409 --> 00:36:06,538
Let me explain to you what happened here and hopefully it'll be clear. Like what did it 

724
00:36:06,539 --> 00:36:11,099
start at a 1,000, why didn't it start at one, what was actually going on? 

725
00:36:11,099 --> 00:36:13,900
What's going on, if I actually look at use counter, 

726
00:36:13,900 --> 00:36:18,170
is remember there is only one counter variable for all objects of the class. 

727
00:36:18,170 --> 00:36:21,329
This first count one comes along and say, "Hey, give me a new object that's an 

728
00:36:21,329 --> 00:36:24,430
incrementor." And we say, "Okay." And we set it's counter 

729
00:36:24,429 --> 00:36:26,649
to be one, right, that's what the constructor did. 

730
00:36:26,650 --> 00:36:30,459
Then I say, "Hey, create for me another counter." And it says, "Okay, here's another 

731
00:36:30,458 --> 00:36:31,679
incrementor over here, 

732
00:36:31,679 --> 00:36:35,058
I'm now setting that counter variable to be a 1,000." 

733
00:36:35,059 --> 00:36:36,220
And you're, like, 

734
00:36:36,219 --> 00:36:36,899
uh-oh, 

735
00:36:36,900 --> 00:36:40,519
that's the same counter variable that count one was using, too. Because now they're 

736
00:36:40,519 --> 00:36:42,219
both sharing the same variable, 

737
00:36:42,219 --> 00:36:44,158
and that variable is now a 1,000. 

738
00:36:44,159 --> 00:36:47,759
The old value of one just got clobbered by this guy because it's referring to 

739
00:36:47,759 --> 00:36:51,050
that same class variable. There's only one counter variable 

740
00:36:51,050 --> 00:36:52,870
among all objects of the class. 

741
00:36:52,869 --> 00:36:57,199
So now when I count the first five values of count one, it's got a 1,000, 

742
00:36:57,199 --> 00:36:59,689
it counts up to a 1,004. 

743
00:36:59,690 --> 00:37:03,579
That 1,004 is shared by all instances of the class. So when I do the 

744
00:37:03,579 --> 00:37:05,380
next five for count two, 

745
00:37:05,380 --> 00:37:08,800
it's starting at a 1,005, and it counts from a 1,005 through 

746
00:37:08,800 --> 00:37:09,580
1,009. 

747
00:37:09,579 --> 00:37:13,170
And then when I do count one again, count one's sharing that 1,009, and that's 

748
00:37:13,170 --> 00:37:16,690
why we get a 1,010 through a 1,014. 

749
00:37:16,690 --> 00:37:21,309
Any questions about that? Um hm. 

750
00:37:21,309 --> 00:37:22,869
[Inaudible.] Oh, yeah, a microphone would be good. 

751
00:37:22,869 --> 00:37:25,689
And let me guess what your question is. If you switch the order, 

752
00:37:25,688 --> 00:37:28,178
yes, you would actually start at zero - or you'd start at one. 

753
00:37:28,179 --> 00:37:31,639
The first constructor would initialize the value at 

754
00:37:31,639 --> 00:37:33,519
a 1,000; 

755
00:37:33,519 --> 00:37:38,079
the second constructor would overwrite the 1,000 with one and then you'd count from one. 

756
00:37:38,079 --> 00:37:42,179
Every once in a while I have this fleeting ten-second ability to 

757
00:37:42,179 --> 00:37:44,699
read minds, and it goes away very quickly. 

758
00:37:44,699 --> 00:37:47,380


759
00:37:47,380 --> 00:37:50,660
So with that said, it's time for something completely different. So any 

760
00:37:50,659 --> 00:37:57,659
questions about instance variables versus class variables? Um hm. [Inaudible.] 

761
00:38:11,858 --> 00:38:15,558
Maybe we should talk about that offline. Because it'd be good to know what particular 

762
00:38:15,559 --> 00:38:16,740
thing you're referring to. 

763
00:38:16,739 --> 00:38:20,208
But most of the time, for a lot of intents and purposes in this class, 

764
00:38:20,208 --> 00:38:23,788
the main time you'll actually see static variables are when they're actually constants. There would 

765
00:38:23,789 --> 00:38:26,989
be very few times when you actually have a static variable that's not a 

766
00:38:26,989 --> 00:38:28,500
final variable, it's not a constant. 

767
00:38:28,500 --> 00:38:32,539
But if you're interested, we could certainly talk about that offline. 

768
00:38:32,539 --> 00:38:35,379
So with that said, there's a funky concept 

769
00:38:35,378 --> 00:38:36,949
called "Javadoc." 

770
00:38:36,949 --> 00:38:40,449
And now you're old enough to learn about Javadoc. And 

771
00:38:40,449 --> 00:38:45,219
all Javadoc really is, is it's basically Java's documentation system. This is just a 

772
00:38:45,219 --> 00:38:48,168
little side point. And the reason why I'm giving this aside to you is you're gonna 

773
00:38:48,168 --> 00:38:51,958
see Javadoc in just a second. We're gonna write another class using Javadoc. 

774
00:38:51,958 --> 00:38:57,029
The difference between regular comments and Javadoc. comments is very simple. 

775
00:38:57,030 --> 00:39:01,390
Javadoc comments start with a slash and two stars instead of just one star, 

776
00:39:01,389 --> 00:39:04,028
but they still end with a star and a single slash. 

777
00:39:04,028 --> 00:39:08,349
So comments that are in that form, that start with a slash and two stars, are called 

778
00:39:08,349 --> 00:39:09,969
Javadoc comments. 

779
00:39:09,969 --> 00:39:12,670
And you'll see what that means in just a second. 

780
00:39:12,670 --> 00:39:16,539
In the comments you can put special tags that are understood by 

781
00:39:16,539 --> 00:39:19,980
the Javadoc system. This is just the system that's gonna generate html 

782
00:39:19,980 --> 00:39:20,639
pages 

783
00:39:20,639 --> 00:39:24,869
that explain what your code does via its comment. So your comments actually serve a 

784
00:39:24,869 --> 00:39:25,768
purpose now. 

785
00:39:25,768 --> 00:39:29,509
Your comments are going to beautifully get transformed into html 

786
00:39:29,510 --> 00:39:32,329
that someone else can look at when they're trying to understand what 

787
00:39:32,329 --> 00:39:35,039
your class does. Instead of having to go through code, they can just look at a 

788
00:39:35,039 --> 00:39:35,998
webpage. 

789
00:39:35,998 --> 00:39:39,678
So there's some special tags, like @ param or @ results, 

790
00:39:39,679 --> 00:39:43,389
that specify what is a parameter of the function you're writing or the 

791
00:39:43,389 --> 00:39:46,749
results if the function's actually returning something. And I'll show you examples of 

792
00:39:46,748 --> 00:39:49,808
that right now. 

793
00:39:49,809 --> 00:39:53,150
So here is a method, and this method might be some method we have that just 

794
00:39:53,150 --> 00:39:56,939
sets the number of units someone earns. So it's called "set units." It takes in some 

795
00:39:56,938 --> 00:39:59,728
number of units as a double, and let's say we have some instance variables, 

796
00:39:59,728 --> 00:40:02,338
some are units earned, which is like how many units you've earned at college so 

797
00:40:02,338 --> 00:40:02,748
far, 

798
00:40:02,748 --> 00:40:04,488
which just gets set units. 

799
00:40:04,489 --> 00:40:08,568
This comment here is a Javadoc comments. It starts with slash, two 

800
00:40:08,568 --> 00:40:11,509
stars, and with a star slash. 

801
00:40:11,509 --> 00:40:14,559
Inside the comment, we still comment what the function does. 

802
00:40:14,559 --> 00:40:18,630
Just like a normal comment, we specify whatever we want to explain to the user. 

803
00:40:18,630 --> 00:40:22,450
Additionally, we can have some special tags, like @ param. And the way @ param 

804
00:40:22,449 --> 00:40:23,129
works is, 

805
00:40:23,130 --> 00:40:26,390
for every one of your parameters you specify @ param. 

806
00:40:26,389 --> 00:40:30,228
Then you specify the name of the parameter, so that's why the words look kind of 

807
00:40:30,228 --> 00:40:33,838
funky, because the name of the parameter is units, that's the next token here, or the 

808
00:40:33,838 --> 00:40:34,679
next 

809
00:40:34,679 --> 00:40:36,139
word is basically units, 

810
00:40:36,139 --> 00:40:40,668
and then what that parameter actually means, the number of units earned. So for human 

811
00:40:40,668 --> 00:40:43,228
reading, and it's not necessarily the most legible thing in the world, but after 

812
00:40:43,228 --> 00:40:45,418
you've seen it for a while you just kind of internalize it, 

813
00:40:45,418 --> 00:40:49,179
but the computer can actually understand that very well and produce some nice 

814
00:40:49,179 --> 00:40:49,858
documentation. 

815
00:40:49,858 --> 00:40:53,958
So let me show you an example of this. It turn out, if you're actually interested, 

816
00:40:53,958 --> 00:40:57,389
all of the ACM libraries, all the libraries that we're using in 

817
00:40:57,389 --> 00:41:01,248
this class, if you want to dig into them, if you go to this 

818
00:41:01,248 --> 00:41:06,558
URL, jtf.acm.org/javadoc/student, because 

819
00:41:06,559 --> 00:41:09,969
it's the student version of the Javadoc, 

820
00:41:09,969 --> 00:41:13,519
you can actually see the Javadoc for all of the ACM libraries. 

821
00:41:13,518 --> 00:41:16,358
So you, until your heart's content, look them over. 

822
00:41:16,358 --> 00:41:20,018
But let's just look at one particular example. Remember random generator that we sort of 

823
00:41:20,018 --> 00:41:23,169
looked at together from the ACM libraries, let me just show you the Javadoc 

824
00:41:23,170 --> 00:41:28,170
for that so you get a sense for what's actually going on. So 

825
00:41:28,170 --> 00:41:32,650
all Javadoc is, is it's html. It brings up a web browser. And what it 

826
00:41:32,650 --> 00:41:35,289
explains in here is here is class random generator. 

827
00:41:35,289 --> 00:41:38,039
This is stuff that got automatically generated based on the 

828
00:41:38,039 --> 00:41:38,919
comments and the code. 

829
00:41:38,918 --> 00:41:42,568
It says, ACM util random generator is actually derived from java 

830
00:41:42,568 --> 00:41:46,338
util random, so it's actually a subclass of java util random. 

831
00:41:46,338 --> 00:41:47,679
And it tells us 

832
00:41:47,679 --> 00:41:51,539
that by saying that actually this public class random generator extends random. So it's 

833
00:41:51,539 --> 00:41:53,199
in fact a subclass. 

834
00:41:53,199 --> 00:41:57,949
And there's a whole bunch of stuff there that explains how this class actually works, and how you get 

835
00:41:57,949 --> 00:42:00,858
random generator.get instance to get a version of it. 

836
00:42:00,858 --> 00:42:04,778
But also what's generated for you, very nicely in these nice html tables, are 

837
00:42:04,778 --> 00:42:05,570
things like, 

838
00:42:05,570 --> 00:42:07,680
"Hey, here's the constructor." 

839
00:42:07,679 --> 00:42:10,969
Constructor has no arguments, that's how you might potentially create a new random 

840
00:42:10,969 --> 00:42:14,358
generator if you wanted to. But, in fact, most of the time what you want to do is get 

841
00:42:14,358 --> 00:42:14,949
instance. 

842
00:42:14,949 --> 00:42:18,318
So for all of the methods it has the name of the method, 

843
00:42:18,318 --> 00:42:19,288
it has, 

844
00:42:19,289 --> 00:42:22,859
basically, the form of the method, which is the method and then whatever 

845
00:42:22,858 --> 00:42:25,599
parameters it takes, and then whatever comments you have 

846
00:42:25,599 --> 00:42:27,398
about that method. 

847
00:42:27,398 --> 00:42:29,618
The other interesting thing about this 

848
00:42:29,619 --> 00:42:32,298
is, 

849
00:42:32,298 --> 00:42:34,699
it tells you which methods are kind of built in 

850
00:42:34,699 --> 00:42:37,458
and which methods actually got 

851
00:42:37,458 --> 00:42:41,418
inherited from your super class. So we mentioned that a random generator is 

852
00:42:41,418 --> 00:42:43,400
an object that extends random. 

853
00:42:43,400 --> 00:42:47,059
Well, some of the methods are actually defined in random generator specifically; 

854
00:42:47,059 --> 00:42:50,119
some of the methods are inherited from this thing called random. It doesn't 

855
00:42:50,119 --> 00:42:52,248
actually make a difference which ones are inherited or not, 

856
00:42:52,248 --> 00:42:55,748
but you get kind of all this text. You get even more text down here. So if you want 

857
00:42:55,748 --> 00:42:58,730
to look at the excruciating details of next boolean, 

858
00:42:58,730 --> 00:43:00,659
that takes in some double p, 

859
00:43:00,659 --> 00:43:04,190
it tells us what the usage is. How do you use next boolean and what are its 

860
00:43:04,190 --> 00:43:07,250
parameters. This is where those @ param little tag 

861
00:43:07,250 --> 00:43:08,230
that was in 

862
00:43:08,230 --> 00:43:12,250
there, there's an @ param tag that says p, and then has this comment, and this automatically 

863
00:43:12,250 --> 00:43:14,568
gets turned into nice html. 

864
00:43:14,568 --> 00:43:18,259
So anytime you have questions about what something in the ACM library does or 

865
00:43:18,259 --> 00:43:21,438
what methods are actually available to you for a particular object, you can 

866
00:43:21,438 --> 00:43:24,879
actually go look at the Javadoc. You don't need to scrounge through code, it's 

867
00:43:24,880 --> 00:43:28,019
actually just html. You can browse in a web browser by going to that URL 

868
00:43:28,018 --> 00:43:30,788
I gave you and search around. But 

869
00:43:30,789 --> 00:43:34,780
the reason for showing you all of this, one, is to let you know that it actually exists, 

870
00:43:34,780 --> 00:43:37,420
the second is we can now put this all together. 

871
00:43:37,420 --> 00:43:42,010
We can create a class that has a bunch of stuff in it that we comment using Javadoc, and 

872
00:43:42,010 --> 00:43:46,459
that also makes use of Strings, and gives you one nice big picture at the end. 

873
00:43:46,458 --> 00:43:50,598
So what I'm gonna do is I'm gonna create a class. 

874
00:43:50,599 --> 00:43:53,730
This class basically is involving a student, 

875
00:43:53,730 --> 00:44:00,730
something that hopefully you're all extremely familiar with at this point. 

876
00:44:06,108 --> 00:44:09,940
So what a student class is going to provide, it's gonna have Javadoc 

877
00:44:09,940 --> 00:44:15,259
comment. So up at the top I have the star, the slash star star, that wasn't a typo, 

878
00:44:15,259 --> 00:44:19,298
that's actually a Javadoc comments, and explains what this whole class does. So it 

879
00:44:19,298 --> 00:44:22,360
says, what I'm gonna do is create a class that keeps track of the student. 

880
00:44:22,360 --> 00:44:25,870
What is information I care about for a student? Well, this is a very simplified form. I care 

881
00:44:25,869 --> 00:44:27,389
about a name of a student, 

882
00:44:27,389 --> 00:44:31,358
their ID No., and how many units they've earned. That's all I care about for right 

883
00:44:31,358 --> 00:44:34,558
now. The book has an example that's a little bit more involved, but I'm gonna give 

884
00:44:34,559 --> 00:44:36,359
you a real simple example here. 

885
00:44:36,358 --> 00:44:39,498
So what are my constructors? How do I create something as a student? Notice 

886
00:44:39,498 --> 00:44:41,679
the class student does not extend anything. 

887
00:44:41,679 --> 00:44:45,210
So all students are just objects, I'm not extending anything else, 

888
00:44:45,210 --> 00:44:46,849
they're just objects. 

889
00:44:46,849 --> 00:44:50,818
One of my constructors for a student says specify the name and ID for the 

890
00:44:50,818 --> 00:44:53,489
student. As a matter of fact, that's gonna be my only constructor, I'm not gonna allow 

891
00:44:53,489 --> 00:44:56,438
someone to do other kinds of things. Give me a name, which is a String, right, it's 

892
00:44:56,438 --> 00:44:58,000
just a piece of text, and 

893
00:44:58,000 --> 00:45:01,760
an integer, which is an ID No. So we'll assume all ID No.'s are 

894
00:45:01,760 --> 00:45:02,719
integers. 

895
00:45:02,719 --> 00:45:06,659
And what it's gonna do is keep track of some information about students 

896
00:45:06,659 --> 00:45:11,199
that are going to be stored as instance variables. Because every student is going to 

897
00:45:11,199 --> 00:45:13,650
have their own information for their name, 

898
00:45:13,650 --> 00:45:16,099
which is just student name that's a String, 

899
00:45:16,099 --> 00:45:18,960
student ID, which is just an integer, and the number of units they've earned so 

900
00:45:18,960 --> 00:45:22,260
far, which I'll keep as a double because there are some schools that actually allow 

901
00:45:22,260 --> 00:45:22,940


902
00:45:22,940 --> 00:45:25,670
half units for whatever classes. They don't here, but 

903
00:45:25,670 --> 00:45:28,829
other places they actually do. So we're just making it a double for generality as opposed to 

904
00:45:28,829 --> 00:45:32,009
having it be an int. So these are all private instance variables. 

905
00:45:32,009 --> 00:45:35,579
No one else outside of a student can get in there and muck with these things. 

906
00:45:35,579 --> 00:45:38,740
The only way we can potentially change them is through the methods that are 

907
00:45:38,739 --> 00:45:40,758
involved in this class. 

908
00:45:40,759 --> 00:45:43,869
We're also gonna have the number of units that are required to graduate (at 

909
00:45:43,869 --> 00:45:45,119
Stanford that's 180), 

910
00:45:45,119 --> 00:45:48,699
and we're going to specify that as a constant that is static and final, 

911
00:45:48,699 --> 00:45:52,039
which means all students share the same number of units to graduate. 

912
00:45:52,039 --> 00:45:55,409
And, unfortunately for you, you can't say, "Hey, you know what, I'm just 

913
00:45:55,409 --> 00:45:57,700
gonna set units to graduate to be like 

914
00:45:57,699 --> 00:46:02,048
ten." Wouldn't that be fun? Like, you take 106a, you take IHUM and you're done. 

915
00:46:02,048 --> 00:46:05,838
Final, yeah, you're not changing this, it's 180 until the cows come 

916
00:46:05,838 --> 00:46:08,139
home. So that's why it's a constant 

917
00:46:08,139 --> 00:46:12,518
as opposed to just being a class variable that's actually modifiable. 

918
00:46:12,518 --> 00:46:15,908
So what are all the things we allow someone to do in here? 

919
00:46:15,909 --> 00:46:19,079
The things we allow someone to do are, for example, to get the name of a 

920
00:46:19,079 --> 00:46:23,459
student. Once I've created a student the student name variable is private. I 

921
00:46:23,458 --> 00:46:26,728
can't actually refer to that variable outside of this class. 

922
00:46:26,728 --> 00:46:29,289
So how do I ask you for your name 

923
00:46:29,289 --> 00:46:32,009
after you've been created? So you were just created as a new student at 

924
00:46:32,010 --> 00:46:32,460
Stanford. 

925
00:46:32,460 --> 00:46:34,130
And, I'm like, "What's your name?" 

926
00:46:34,130 --> 00:46:38,030
And I want to kind of come over and get a little close and be like, "Hey, can I touch your 

927
00:46:38,030 --> 00:46:39,880
variables?" And you're, like, 

928
00:46:39,880 --> 00:46:43,259
"No, man, this is wrong." 

929
00:46:43,259 --> 00:46:46,398
So I'm, like, "Oh, what do you have that's 

930
00:46:46,398 --> 00:46:48,449
public? Oh, get 

931
00:46:48,449 --> 00:46:51,688
your name." What's your name? Aki. 

932
00:46:51,688 --> 00:46:55,098
Aki. So I get back a String Aki from your object. 

933
00:46:55,099 --> 00:46:58,778
I can't actually touch any of the private parts. 

934
00:46:58,778 --> 00:47:00,438


935
00:47:00,438 --> 00:47:04,148
ID number, same kind of deal. I can't go in and see what your ID - like, I'm not gonna 

936
00:47:04,148 --> 00:47:07,838
go into your wallet and pull out your ID card and be, like, "Oh, ID number, let me get out my magic 

937
00:47:07,838 --> 00:47:09,398
marker and change it." 

938
00:47:09,398 --> 00:47:12,528
The only way I can get it is by asking you for it, and if you've made that 

939
00:47:12,528 --> 00:47:15,858
public then I can get that information from you by 

940
00:47:15,858 --> 00:47:16,998
making the method public. 

941
00:47:16,998 --> 00:47:20,018
A few other things. Now, here are some funky things. 

942
00:47:20,018 --> 00:47:23,998
Set units I actually made to be public, which means I can allow someone, even 

943
00:47:23,998 --> 00:47:25,398
though 

944
00:47:25,398 --> 00:47:29,518
the number of units earned is a private variable, so I can't directly 

945
00:47:29,518 --> 00:47:31,779
just go in and modify the variable directly, 

946
00:47:31,780 --> 00:47:35,180
I can call set units and give it some number and it will just change the 

947
00:47:35,179 --> 00:47:38,428
number of units you have. 

948
00:47:38,429 --> 00:47:41,559
I can ask you for how many units you have. So this is a dangerous thing, if you 

949
00:47:41,559 --> 00:47:43,819
think about it. And that's the kind of thing you want to think about, in terms of 

950
00:47:43,818 --> 00:47:47,498
good programming style, what do you make public and what do you make private. And if red 

951
00:47:47,498 --> 00:47:49,389
flags go off because you're making things public 

952
00:47:49,389 --> 00:47:50,929
that shouldn't really be public, 

953
00:47:50,929 --> 00:47:53,119
that's something you want to think about for style. 

954
00:47:53,119 --> 00:47:54,079
Get units, 

955
00:47:54,079 --> 00:47:57,269
this just returns how many units you've actually earned as a double. And, 

956
00:47:57,268 --> 00:47:59,908
again, you can see we have a return 

957
00:47:59,909 --> 00:48:03,469
tag here for the Javadoc that says the number of units the student has earned is 

958
00:48:03,469 --> 00:48:05,608
the things we're actually returning. 

959
00:48:05,608 --> 00:48:08,659
A couple other things real quickly, and then I'll show you that we're gonna make use 

960
00:48:08,659 --> 00:48:11,858
of this class. 

961
00:48:11,858 --> 00:48:14,818
There's something called "increment units," which basically just says, every time 

962
00:48:14,818 --> 00:48:18,058
you take a class I want to increment the total number of units you have by the 

963
00:48:18,059 --> 00:48:19,760
number of units in this class. So I 

964
00:48:19,760 --> 00:48:22,839
pass in some value for additional units, and I increment your units earned by 

965
00:48:22,838 --> 00:48:24,228
additional units. 

966
00:48:24,228 --> 00:48:27,908
Having enough units, that's a predicate method, it returns a boolean. And 

967
00:48:27,909 --> 00:48:30,959
what it basically returns is the number of units earned greater than or equal 

968
00:48:30,958 --> 00:48:33,739
to this constant for how many units you need to graduate. 

969
00:48:33,739 --> 00:48:36,358
And last, but not least, and this is a critical one, 

970
00:48:36,358 --> 00:48:40,708
all classes you write should have a method in them called "two String." 

971
00:48:40,708 --> 00:48:43,049


972
00:48:43,050 --> 00:48:45,310
What two String actually means 

973
00:48:45,309 --> 00:48:51,228
is get me some textual version of what's the information in this class. So 

974
00:48:51,228 --> 00:48:53,239
two String for a student 

975
00:48:53,239 --> 00:48:58,249
returns back a String, which is the students name, plus their ID number in 

976
00:48:58,248 --> 00:49:01,689
parenthesis. It does not necessarily specify how many units they've earned, that's fine. It 

977
00:49:01,690 --> 00:49:04,940
doesn't need to include all the information that's actually in the object. 

978
00:49:04,940 --> 00:49:08,510
What it just should specify in a nice textual way is the information that you 

979
00:49:08,510 --> 00:49:12,789
care about displaying about an object if someone wants to display it. 

980
00:49:12,789 --> 00:49:16,179
But all classes you write that are not programs. So something that 

981
00:49:16,179 --> 00:49:19,499
extends Console program doesn't need this, any other classes should have a two 

982
00:49:19,498 --> 00:49:20,948
String. And 

983
00:49:20,949 --> 00:49:22,289
that's the whole class. 

984
00:49:22,289 --> 00:49:26,679
So any questions about this class or should I just show you how we use it? 

985
00:49:26,679 --> 00:49:29,369
Let me show you how we use it. Well, it's Stanford, you're at Stanford so we've 

986
00:49:29,369 --> 00:49:32,390
gotta have a program called Stanford that actually creates students. 

987
00:49:32,389 --> 00:49:36,168
So Stanford is a program that extends Console program. 

988
00:49:36,168 --> 00:49:38,878
And, again, we have something to increase the font size. But we're gonna 

989
00:49:38,878 --> 00:49:42,298
create some new student, 

990
00:49:42,298 --> 00:49:43,360
Ben Newman. 

991
00:49:43,360 --> 00:49:45,440
And that's pronounced 

992
00:49:45,440 --> 00:49:48,219


993
00:49:48,219 --> 00:49:50,438
stud. 

994
00:49:50,438 --> 00:49:54,228
So we have some student, stud, that's a new student. And remember we need to 

995
00:49:54,228 --> 00:49:57,498
specify the name and ID number. to initialize that student. Ben Newman, your 

996
00:49:57,498 --> 00:50:00,728
ID number's now 1,001. 

997
00:50:00,728 --> 00:50:03,899
I'm gonna set your estimated number of units 

998
00:50:03,900 --> 00:50:05,059
at 179. 

999
00:50:05,059 --> 00:50:09,069
And then I want to write out how many units you have. I cannot refer to your 

1000
00:50:09,068 --> 00:50:12,259
name and number of units directly, I need to ask 

1001
00:50:12,259 --> 00:50:15,559
what's the object, that's the receiver of the message. And the message 

1002
00:50:15,559 --> 00:50:18,639
I'm gonna send is the name of the method I just wrote, stud.get name, 

1003
00:50:18,639 --> 00:50:21,450
that returns to me a String, which is the name that's Ben Newman. 

1004
00:50:21,449 --> 00:50:22,848
I append to that has, 

1005
00:50:22,849 --> 00:50:25,620
I append to that the number of units Ben has, 

1006
00:50:25,619 --> 00:50:29,150
and then add the word units at the end of it and write that out to the screen. 

1007
00:50:29,150 --> 00:50:32,579
Then I want to see if Ben can graduate. So, again, I print out Ben's name, 

1008
00:50:32,579 --> 00:50:34,630
if he can graduate, his status 

1009
00:50:34,630 --> 00:50:38,769
by calling "has enough units." That will return true or false, that will write true or 

1010
00:50:38,769 --> 00:50:39,858
false out 

1011
00:50:39,858 --> 00:50:42,598
onto the screen as part of the String. 

1012
00:50:42,599 --> 00:50:46,039
Then Ben takes CS106a. So 

1013
00:50:46,039 --> 00:50:49,409
Ben takes CS106a, is what's gonna get written out, and then I 

1014
00:50:49,409 --> 00:50:52,929
increment Ben's units by five because he's now taking CS106a, a five 

1015
00:50:52,929 --> 00:50:56,880
unit class. And then I ask again can he graduate. 

1016
00:50:56,880 --> 00:51:01,048
And if he has enough units to graduate, I'll say, rock on, 

1017
00:51:01,048 --> 00:51:06,489
and write out the String version of the object that I have by calling two String. 

1018
00:51:06,489 --> 00:51:08,409
So when I actually run this 

1019
00:51:08,409 --> 00:51:10,358
all here's what I get. 

1020
00:51:10,358 --> 00:51:16,048
I'm 

1021
00:51:16,048 --> 00:51:21,188
running. I'm running. I want to run Stanford. It's just that easy to run 

1022
00:51:21,188 --> 00:51:25,278


1023
00:51:25,278 --> 00:51:27,268
Stanford. 

1024
00:51:27,268 --> 00:51:30,668
So Ben Newman has 179.0 units, right, because it's a double so 

1025
00:51:30,668 --> 00:51:32,608
it puts that .0 at the end of it. 

1026
00:51:32,608 --> 00:51:36,119
Ben Newman can graduate. We get false back because he doesn't have enough units to 

1027
00:51:36,119 --> 00:51:39,528
graduate. But that 179 is stored inside the object, 

1028
00:51:39,528 --> 00:51:43,188
that stud object has the 179. Ben takes CS106a, so 

1029
00:51:43,188 --> 00:51:47,168
we increment that object's number of units by five, by calling the appropriate 

1030
00:51:47,168 --> 00:51:50,558
method, and then Ben Newman can graduate is now true because he's got 

1031
00:51:50,559 --> 00:51:51,589
184 units. 

1032
00:51:51,588 --> 00:51:53,860
And, finally, when we say, "Rock On," 

1033
00:51:53,860 --> 00:51:58,010
and we call two String, what two String gives us back is the name 

1034
00:51:58,010 --> 00:52:02,660
plus the ID number inside paren. So it says, "Rock On Ben Newman paren number 

1035
00:52:02,659 --> 00:52:04,629
1001." 

1036
00:52:04,630 --> 00:52:07,548
Any questions about that? 

1037
00:52:07,548 --> 00:52:09,719
Alrighty. Then have a good weekend and I will see you on Monday.